Photodiodes comprise of multiple radiation sensitive junctions formed in semiconductor material. Within a photodiode, charge carriers are created by light that illuminates the junction and photo current is generated dependent upon the degree of illumination. Similarly, a photodiode array comprises a plurality of light sensitive spaced-apart elements, comprising of a semiconductor junction and a region of high response where the photo-generated charge carriers are collected. Photodiodes are used in various applications including, but not limited to, optical position encoding, and low light-level imaging, such as night photography, nuclear medical imaging, photon medical imaging, multi-slice computer tomography (CT) imaging, radiation detection and ballistic photon detection.
Photodiodes are characterized by certain characteristics, such as electrical, optical, current (I), voltage (V), and noise. Electrical characteristics of photodiode dominantly include shunt resistance, series resistance, junction capacitance, rise or fall time and frequency response. Noise in photodiodes is generated by a plurality of sources including, but not limited to, thermal noise, quantum or photon noise, and flicker noise.
FIG. 1 shows a cross sectional view of a prior art wavelength sensitive photodiode device 100. The device 100 typically comprises two PN junctions: first junction 105 comprised of shallow boron P+ 120 implanted into N deep driven-in layer 125 and second junction 110 comprised of phosphorus deep diffusion of the N-layer 125 into a high resistivity P-type silicon substrate wafer 103. Appropriate cathode 101 and anode 102, 104 metal contacts are formed and revealed through antireflective layers 115. Electrode terminals comprising cathode 101 and anode 102, in combination, form output terminals of a first photodiode PD1 associated with the first junction 105, while cathode 101 and anode 104 form output terminals of a second photodiode PD2 associated with the second junction 110. However, one of the disadvantages of such prior art photodiode device 100 is that the doping concentration of the N-layer 125 is typically high and on the order of 1×1016/cm3, resulting in low minority carrier lifetime and low quantum efficiency of the first photodiode 105.
FIG. 2 shows typical spectral responsivity of 0.15 A/W prior art first photodiode PD1, associated with the first PN junction 105 of FIG. 1, at 660 nm wavelength. Referring now to FIGS. 1 and 2 simultaneously, curve 205 represents the spectral sensitivity derived from the first PN junction 105 (photodiode PD1), which has a peak p1 at a shorter wavelength side. Curve 210 represents spectral sensitivity derived from the second PN junction 110 (photodiode PD2), which has a peak p2 at a longer wavelength side. It can be observed from FIG. 2 that the responsivity of the second photodiode PD2 is higher, as depicted by curve 210, in comparison to the first photodiode PD1.
Accordingly, there is need in the prior art for an improved wavelength sensitive photodiode that employs high quality n-type layer with relatively lower doping concentration. Specifically, there is need in the prior art for novel structure of a photodiode that enables high minority carrier lifetime and high quantum efficiency with improved responsivity at multiple wavelengths.